Method of forming a fiber-reinforced ceramic matrix composite

ABSTRACT

This invention pertains to a method of forming a fiber-reinforced ceramic matrix composite comprising: (a) impregnating a ceramic fiber coated with at least one layer binary coating comprised of boron nitride and silicon nitride wherein the silicon nitride is applied over the boron nitride with a preceramic composition comprising a curable preceramic polymer; (b) forming the impregnated fibers into a desired shape; (c) curing the formed impregnated fibers; (d) heating the cured impregnated fibers of (c) to a temperature of at least 1000° C. in an inert atmosphere for a time effective to convert the preceramic polymer to a ceramic.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a division of application Ser. No.09/198,979 filed Nov. 24, 1998, entitled “Ceramic Matrix Composites.”

FIELD OF THE INVENTION

[0002] This invention pertains to ceramic matrix composites thatcomprise ceramic fibers coated with at least one binary coatingcomprised of boron nitride (BN) and silicon nitride (Si₃N₄) withinceramic matrix. The ceramic matrix is derived from curable preceramicpolymers. The composites can be formed into complex shapes which havegood oxidation resistance at high temperature, high flexural strengthand are resistant to moisture.

BACKGROUND OF THE INVENTION

[0003] It is well known that for ceramic matrix composites, interfacialbonding between the reinforcing fiber and matrix controls the mechanicalproperties of the composite. In many ceramic matrix compositesreinforced with siliconoxycarbide fibers, carbon coatings on the fiberhave been shown to control interfacial bonding between fiber and matrixto produce desired mechanical properties. It is possible to apply thecarbon coating to the fiber before fabricating the composite, or toprocess the composite for short durations at approximately 1000° C.under non-oxidizing conditions to produce a thin carbon layer on thefiber.

[0004] Unfortunately, the use of these ceramic matrix composites in hightemperature (>500° C.), oxidizing environments tends to degrade strengthand strain tolerance. In some cases it has been shown that the use of BNcoatings in place of the carbon coating between the fiber and the matrixsubstantially improves the oxidative stability of the ceramic matrixcomposite. For example, U.S. Pat. No. 4,642,271 to Rice discloses aceramic fiber composite material comprised of boron nitride (BN) coatedceramic fibers embedded in a ceramic matrix. U.S. Pat. No. 5,198,302discloses silicon nitride reinforcing fibers provided with a protectivesurface coating comprising a BN base layer and optionally an aluminaovercoating. U.S. Pat. No. 5,354,602 to Stranford et al. discloses theuse of BN coated fibers in a matrix of black glass ceramic. U.S. Pat.No. 5,707,471 to Petrak et al., discloses the fibers coated with carbon,boron nitride, silicon carbide, silicon nitride, aluminum nitride andcombinations of these.

[0005] It has now been found that ceramic matrix composites, withpolymer derived matrices, that use BN coated fibers are susceptible tomoisture corrosion at low temperatures (100° C).

[0006] U.S. Pat. Nos. 5,580,643 and 5,202,059 disclose duplex coatedceramic filler materials wherein the filler material may be a fiber andthe coatings are boron nitride (BN) and silicon carbide (SiC). However,this duplex coating does not provide the benefits of this invention,including the moisture resistance.

[0007] Kowbel et al. in “A Chemical Vapor Deposition (CVD) BN—Si₃N₄Interfacial Coating for Improved Oxidation Resistance of SiC—SiCComposites”, Journal of Materials Synthesis and - Processing, Vol. 3,No. 2 (1995) pp. 121-131 disclose the use of a mixture of BN and Si₃N₄to coat SiC fibers. However, as can be seen in FIG. 11, these compositeshave about the same flexural strength as a BN coated fiber.

[0008] It is an object of this to provide ceramic matrix compositeswhich contain coated fibers wherein the coating comprises at least onebinary layer comprised of boron nitride and silicon nitride.

SUMMARY OF THE INVENTION

[0009] This invention pertains to a method of forming a fiber-reinforcedceramic matrix composite comprising:

[0010] (a) impregnating a ceramic fiber coated with at least one layerbinary coating comprised of boron nitride and silicon nitride whereinthe silicon nitride is applied over the boron nitride with a preceramiccomposition comprising a curable preceramic polymer;

[0011] (b) forming the impregnated fibers into a desired shape;

[0012] (c) curing the formed impregnated fibers;

[0013] (d) heating the cured impregnated fibers of (c) to a temperatureof at least 1000° C. in an inert atmosphere for a time effective toconvert the preceramic polymer to a ceramic. The ceramic matrixcomposites containing the coated fibers maintain flexural strength whenexposed to moisture.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 represents a bar chart representation of the interlaminarshear strength of ceramic matrix composites containing a fiber having aBN interfacial coating before and after rain/engine thermal cycleexposure.

[0015]FIG. 2 represents a bar chart representation of the interlaminarshear strength of ceramic matrix composites containing a fiber having aninterfacial coating of this invention before and after rain/enginethermal cycle exposure.

DETAILED DESCRIPTION OF THE INVENTION

[0016] This invention pertains to matrix composites comprising ceramicfibers having coated thereon at least one binary layered coatingcomprised of boron nitride (BN) and silicon nitride (Si₃N₄). The ceramicfibers which may be used in this invention comprise high-modulus fiberswhich are compatible with the coatings and matrices described herein andwhich can withstand the polymer impregnation process. These fibers arewell known in the art and many are commercially available. Examples ofsuitable fibers include those of silicon carbide, silicon nitride,silicon carbide deposited on a carbon core, aluminum borate, aluminumoxide, silicon oxide, silicon carbide containing titanium, siliconoxycarbides, silicon oxycarbonitrides, carbon and the like. Generally,such fibers should have a modulus of greater than 100 GPa, preferablygreater than 150 GPa. These fibers may contain any desirable number offilaments per tow and have a diameter in the range of about 5 μm toabout 500 μm.

[0017] Examples of specific fibers include, but are not limited to,silicon carbide fibers with a diameter in the range of 10-20 μmmanufactured by Nippon Carbon (Nicalon®); fibers comprising siliconcarbide deposited on a carbon core with a diameter of about 143 μmmanufactured by Avco; alumina-boria-silica fibers with a diameter ofabout 10-12 μm manufactured by 3M; Al₂O₃ fibers with a diameter of about20 μm manufactured by DuPont; SiO₂ fibers with a diameter of about 8-10μm manufactured by J. P. Stevens; Al₂O₃—SiO₂ fibers with a diameter inthe range of about 9-17 micrometers manufactured by Sumitomo; siliconcarbide fibers containing titanium with a diameter in the range of 8-10μm manufactured by Ube (Tyranno®); silicon carbide fiber with a diameterin the range of 6-10 μm manufactured by Avco; silicon oxycarbonitridefibers with a diameter in the range of about 10-15 μm; silicon carbidefibers with a diameter in the range of about 10-15 μm manufacture by DowComing (Sylaramic™); silicon nitride fibers such as those produced byTonen or Rhone Poulanc and Al₂O₃—ZrO₂ fibers with a diameter of about 20μm manufactured by DuPont. These commercial fibers may be supplied witha surface sizing. It is preferable to remove the sizing prior to theapplication of the coating.

[0018] Although any of the above fibers are functional, those preferredherein comprise ceramic fibers of silicon and carbon and optionallyoxygen. Especially preferred are silicon oxycarbide fibers (Nicalon® andTyranno®) and silicon carbide fibers (Nicalon® and Sylramic™).

[0019] The ceramic fibers are coated with at least one binary coatingcomprised of boron nitride and silicon nitride. The binary coating isapplied to the ceramic fiber by first applying a layer of boron nitrideonto the ceramic fiber followed by the application of a coating ofsilicon nitride over the boron nitride coating. Additional binarycoatings of boron nitride and silicon nitride may be applied to theceramic fiber in the same manner. However, it is preferred to produce afiber having one binary coating of boron nitride and silicon nitride.Each individual coating thickness (BN or Si₃N₄) is typically in therange of 0.02 to 1 μm, preferably from 0.05 to 0.3 μm. The coatings maybe deposited by any means known in the art such as chemical vapordeposition or by coating with polymer precursors followed by pyrolysis.

[0020] Optionally, additional coatings may be applied over a singlebinary coating of BN/Si₃N₄ or between multiple layers of the binarycoating (i.e. over the Si₃N₄ but under the next BN/Si₃ N₄ coating).These additional coatings may be any known interface coating such ascoatings of carbon, silicon carbide, and aluminum nitride, preferablysilicon carbide.

[0021] Preferably the coatings are applied by chemical vapor depositiontechniques. For example, boron trichloride and ammonia are heated to atemperature of 980° C. to 1000° C. at a pressure in the range of 0.2torr to 1.0 torr to produce the boron nitride coating. To produce thesilicon nitride coating, silicon tetrachloride and ammonia are used atthe same deposition conditions. By products are removed from thedeposition system down stream and away from the coated fiber.

[0022] The coated fibers may be used in nearly any length and may bearranged in the matrix in nearly any manner desired. Generally, thefibers are essentially continuous and are either alignedunidirectionally, woven as a 2-dimensional fabric or shaped as a3-dimensional reinforced preform. It is preferable to heat the coatedfiber to about 1100° C. to 1300° C., preferably about 1200° C., prior toits use in preparing the composite. Preferably the fiber is heated underatmospheric pressure in a nitrogen environment in a carbon-linedfurnace.

[0023] The matrices are derived from curable preceramic polymers. Theexpression “curable” is used herein to describe polymers which can bedeep section infusibilized (cured) in the composite under moderateconditions by means such as mild heat, radiation, curing catalysts, orcuring agents. This curability prevents the composite from delaminatingduring pyrolysis.

[0024] Any curable preceramic polymer may be used in the presentinvention. Preferable curable preceramic polymers are organosiliconpolymers selected from the group consisting of polysiloxanes,polysilazanes, polysilanes, polycarbosilanes, polysilsesquioxanes,polymetallosiloxanes and others, preferably polysilazanes. These curableorganosilicon preceramic polymers are well known in the art and aredescribed in U.S. Pat. Nos. 5,447,893 and 5,707,471 to Petrak et al.,commonly owned, herein incorporated by reference for the teaching ofcurable organosilicon preceramic polymers. Suitable polysilazanesinclude, but are not limited to hydridopolysilazanes,silacyclobutasilazanes, boron modified hydridopolysilazanes andvinyl-modified hydridopolysilazanes.

[0025] In addition to above fibers and curable preceramic polymers, thecomposites may also contain fillers. Suitable fillers are known in theart and may be exemplified by, but not limited to, powders, whiskers orparticulates of metal oxides such as Al₂O₃, SiO₂, silicon carbide,silicon nitride, silicon hexaboride, aluminum nitride, boron nitride,boron carbide, titanium boride, boron, titanium carbide, aluminumnitride and others. The preferred fillers are boron nitride, siliconcarbide and silicon nitride. Such fillers are generally included inamounts up to about 65 volume percent based on the volume of the matrixmaterial, preferably from 5 to 50 volume percent.

[0026] The composites herein may be produced by polymer impregnation.This process comprises first impregnating the coated fibers with aliquid preceramic mixture comprising the curable preceramic polymer andoptionally, fillers. The preceramic mixture can be formed by either asolution or melt route.

[0027] In the solution route, the curable preceramic polymer and fillersare mixed in an organic solvent. The preferred solvents are those with alow vaporization point, preferably <125° C., at atmospheric pressure tofacilitate removal from the impregnated fibers and those with less thanabout 1 wt % water. Examples of suitable organic solvents includealiphatic hydrocarbons such as hexane, heptane and others and aromatichydrocarbons such as benzene, toluene and others. The concentration ofcurable preceramic polymer in solution can be varied over a wide rangewith higher concentrations generally resulting in larger amounts of thepreceramic polymer impregnating the fiber. Preferably, concentrations inthe range of about 20 to about 60 weight percent are employed herein.

[0028] In the melt route, the curable preceramic polymer is heated to atemperature above its melting point yet below its curing temperature inan inert environment. Fillers may also be mixed in the molten polymer ifdesired.

[0029] The coated fibers are then impregnated with the preceramicmixture by any convenient means. For instance, the fibers can beimmersed in the mixture, sprayed with the mixture, held under a streamof the mixture and others. The impregnated fibers can additionally bemanipulated to uniformly distribute the matrix mixture in the fibers.Following impregnation, any excess matrix mixture on the fibers isallowed to drain off.

[0030] If the solution route to the preceramic mixture is used, thesolvent is allowed to evaporate. Generally, any practical method such asair evaporation at room temperature or the use of vacuum or mild heatmay be used. The resultant fibers which have been impregnated and thesolvent evaporated are commonly called a “pre-preg”.

[0031] If the melt route to the preceramic mixture is used, theimpregnated fibers can merely be cooled to form the “pre-preg”.Alternatively, however, the melt impregnated fibers may be formed priorto cooling by a process such as filament winding or pulltrusion. Whenthese fibers are cooled, they can be immediately cured and fired as setforth below.

[0032] The pre-preg is subjected to externally applied pressure whileheating to form the composite into the desired shape and causeuniformity of resin and the coated fibers. Generally, this isaccomplished by pressing the pre-preg into a mold at a temperature andpressure which allows the resin to flow throughout the mold. Thepressing conditions generally used therein include temperatures in therange of about 150° C. to about 300° C., pressures in the range of about6.9 to 6,900 kPa (1 to 1000 psi), and times in the range of about 30minutes to about 15 hours. Pressing at about 175° C. to 230° C., 1380 to2760 kPa (200-400 psi) and 6 to 15 hours generally provides satisfactoryresults. Temperatures and pressure which result in resin being forcedout of the mold should be avoided.

[0033] It should be noted that if a 3-dimensional (3-D) shape isdesired, the above steps are often altered. To manufacture 3-D objectsby this process, one generally first forms the coated fiber into thedesired shape and then impregnates the formed coated fiber with thepolymer mixture. The impregnated fibers are then pressed, cured, andfired as set forth herein.

[0034] The formed pre-preg is next infusibilized (cured) to insurecomplete or nearly complete crosslinking such that deformation onpyrolysis will not occur. Any method which produces the desired resultmay be used so long as the temperature does not cause ceramification. Apreferred method comprises heating at 250° C. to 300° C. for up to 16hours, preferably for 2 to 16 hours. This infusibilization (curing) stepmay be performed in the mold under pressure or it may be accomplished ina conventional oven under nitrogen without any applied pressure.

[0035] The pressed and cured product (green composite or molded part) isthen fired in a furnace to a temperature of at least 1000° C. in aninert atmosphere until the product ceramifies. It is preferred that thegreen composite be fired at a temperature of about 1200° C. to 1300° C.Preferably, the cured product is slow fired wherein the composite isheated in a slow (e.g. 2° C./min.), stepwise, linear fashion until themajority of any higher boiling volatiles present escape the compositeafter which time the temperature can be quickly raised to the ultimatefiring temperature.

[0036] After completion of the firing process the composite is cooled to<100° C. When cooled, the resulting material is uniform, hard, strongfiber reinforced composite. The volume percentage of coated fibers inthe resulting composite can vary over a wide range depending on thedesired use. Generally, it is preferred that about 10 to 65 vol % of thecomposite is fiber.

[0037] The composites formed by the above process are generally quiteporous. Since it may be preferred to produce dense objects, thecomposites may be reimpregnated and pyrolyzed until the desired densityis achieve. This is accomplished by merely impregnating the compositewith the curable preceramic polymer (without filler) as describe above(e.g. solution route or melt route), curing the reimpregnated compositeand then firing. This reimpregnation process is then repeated until acomposite with the desired density and strength is achieved.

[0038] The composites produced herein have many desirable propertiessuch as high flexural strength, good oxidation resistance at hightemperatures, high strength and toughness, a wide range of dielectricproperties and moisture resistance (as measured by retention of flexuralstrength and/or shear strength after exposure to moisture).

[0039] So that those skilled in the art can understand and appreciatethe invention taught herein, the following examples are presented, itbeing understood that these examples should not be used to limit thescope of this invention found in the claims.

EXAMPLES Example 1-Comparative

[0040] Matrix Precursor Formulation

[0041] The matrix precursor was prepared by mixing the filler powder(Table 1) with a boro hydridopolysilazane polymer (Boro-HPZ) in toluene.In each case the filler and Boro-HPZ (i.e. solids) was 50% of the slurryby weight. When BN was used as the filler it was 20% of the solids. WhenSiC powder was used as the filler, it was 25% of the solids. Mixing ofthe matrix slurry was done by ball milling the total mixture for twohours in a plastic jar with 0.25 inch diameter SiC balls. The plasticjar was 500 cm volume and 200 g of SiC balls were used during the mixingoperation. The total weight of the slurry produced was 150 g.

[0042] Prepreg Preparation

[0043] The preparation of prepreg was done by pouring the matrix slurryover the coated cloth and gently rubbing the slurry into the woven clothto assure penetration of the slurry into the fiber tows. The saturatedcloth was then run through a set of metal rolls to remove excess matrixprecursor. The piece of cloth was then suspended in a hood to evaporatethe toluene solvent.

[0044] After typically one hour, the prepreg was drapeable and slightlytacky. At that stage, the solids content of the matrix precursor was 40to 50% of the weight of the prepreg total.

[0045] Preparation of unidirectional tape was done by dripping thematrix slurry on the coated fiber tape which was wound on a one meterdiameter drum. This was done while using a rotating drum which tended tospread the slurry uniformly over the tape. The tape had previously beenwound to carefully place a monolayer of fiber tow on the drum.

[0046] After the solvent was evaporated, approximately one hour, therotating drum was stopped and the tape was removed by cutting one timeto create an impregnated tape approximately 3.14 meters long. The tapeis then ready for laying-up into a composite.

[0047] Composite Molding Procedure

[0048] The procedure to mold the composites was to cut the prepreg clothtest specimens into approximately 16.5 cm×16.5 cm pieces. Eight pieces(plies) of prepreg were cut using a razor knife. The plies were stackedas warp direction aligned symmetrical eight ply composites in the caseof woven cloth composites. Most typically an 8 harness satin weave clothwas used for these composites. The satin weave produces high volumefraction fiber compared to plain weave.

[0049] In the case of composites formed by the use of prepreg ini-tapes,the tapes were stacked as either one direction reinforced composites or0/90 lay-up where the direction of the tapes were alternating. The 0/90architectures were also stacked to be symmetrical about a mid-plane.

[0050] Once the prepreg plies were stacked they were ready for vacuumbagging. This consisted of an aluminum plate 30 cm×50 cm, one layer ofpeel ply, the stack of prepreg plies, 25 another peel ply, a second 18cm×18 cm aluminum plate and a sheet of Vac-Pac UHT-650-XT bonded to thelarger aluminum plate using a high temperature tape (Schnee-Morehead5158). A vacuum port and breather fabric were introduced through theVac-Pac sheet.

[0051] Molding was done by placing the cull plate and vacuum bag in awarm molding press that was preheated to 120° C. The vacuum bag wasloaded to produce a stress on the stack of plies of 689.5 MPa (100 psi).The conditions of 120° C. and 689.5 MPa (100 psi) were maintained for 30minutes. The temperature was then increased to 180° C. for one hour andraised again to 260° C. for 2 hours. The pressure was raised to 1034 MPa(150 psi) during the 260° C./2 hour hold. The press was cooled and thepressure was allowed to slowly release due to cooling the press.

[0052] The composites were then weighed and checked for dimensions.Excess resin that was squeezed from the plies was removed from the edgeof the panel. A 20 hour post-cure cycle was also completed by heating to285° C. in a nitrogen atmosphere.

[0053] Pyrolysis and Composite Densification

[0054] The post-cured composite panels were heated in a furnace with anitrogen atmosphere to 1000° C. at 100° C. per hour. The temperature washeld for one hour. The temperature was then raised to 1200° C. in onehour and held for 2 hours. After cooling to less than 100° C. the panelswere removed from the furnace and inspected. Typically, the panels wouldnot change dimensions but the composite would loose approximately 9% ofits weight.

[0055] That weight loss produced approximately 30% open porosity in thepyrolyzed composite. In order to reduce the open porosity, the compositepanels were then impregnated with a 50% solution of HPZ polymer intoluene. The impregnation was done at room temperature by placing thepanel in an evacuated chamber and introducing the HPZ solution. Once thepart was submerged in the solution, the vacuum was released and thechamber pressure was raised to ambient pressure.

[0056] The panels were permitted to remain in the solution for 30minutes; then they were removed from the solution and placed in anexhaust hood to evaporate the toluene solvent for at least one hour.After the solvent was removed the panel was heated again to 1200° C. inflowing nitrogen using the same heating schedule described above. Thisreimpregnation and pyrolysis cycle was repeated until the compositesshowed an open porosity level that was measured to be 6% or less using aliquid immersion method. As few as 10 or as many as 17 pyrolysis cycleswere required to reduce the open porosity to less than six percent.

[0057] Five composite panels were fabricated using this process method.They are identified as C1-a, C1-b, C1-c, C1-d and C1-e. All panels wereprepared using a Boro-HPZ prepreg resin and CG Nicalon™ fiberreinforcement. But each panel used a different combination of interfacecoating and filler as shown in Table 1. Panels C1-a through C1-e usingCG Nicalon™ fiber in the form of an 8 harness satin woven cloth.

[0058] All panels exhibited good three-point flexure strength at roomtemperature in the as made condition. However, panels 1-a and 1-b, whichused a BN interface coating, showed relatively poor flexure strengthafter being subjected to boiling distilled water for 24 hours. PanelsC1-c, C1-d and C1-e had a carbon interface coating and produced betterretention of flexure strength than panels C1-a and C1-b after the 24hour water boil test. However, it is known that the carbon interface isalso susceptible to oxidation at elevated temperatures an thereforewould be less effective in structural applications. TABLE 1 Propertiesof CG Nicalon ™ Fiber Composites with various filler and interfacecoatings. Flexural Strength, MPa Example Fiber Coating Filler BeforeAfter % Retention C1-a BN BN 368.5 104.2 28.3 C1-b BN SiC 400.2 93.823.4 C1-c C BN 345.0 144.9 42.0 C1-d C None 387.8 317.4 81.9 C1-e C SiC263.6 258.1 97.7

Example 2

[0059] Composite panels made from the same matrix and fiber using thesame process as described in Example 1 were made with a non-woven fiberarchitecture and binary interface coatings. Panel 2 was prepared as a0/90 architecture composite from a unidirectional tape. The interfacecoating was one binary coating of 0.3 micrometers of BN and 0.2micrometers of Si₃N₄ deposited on the fiber. Table 2 lists theproperties of a 12-ply composite that used 20% BN as the matrix filler.Specimen 2-a showed as made average three-point flexure strength to be431.2 MPa (62.5 ksi). Specimen 2-b was subjected to the 24 hour waterboil test as described in Example 1. Speciment 2-b showed an average3-point flexure strength of 318.1 MPa (46.1 ksi). That level of strengthretention after water exposure, 73.4% was substantially improvedcompared to the BN interfaced coated materials produced in Example 1.

Example 3

[0060] Specimens taken from panel 3 were fabricated using aunidirectional tape where all the fibers were aligned in one direction.Otherwise, this panel was processed the same as the panels in Example 1.The interface coating used in panel 3 was six binary coatings of BN andSi₃N₄. The total coating thickness though was approximately 0.7micrometers. This panel also used BN as the matrix filler. As it shownin Table 2, specimens 3-a and 3-b show that the water boil exposureproduced improved flexure strength compared to the as made strength.Specimen 3-c demonstrates that this panel retains good four-pointflexure strength at 1000° C. in air. The carbon interface compositesshow rapid reduction in strength when exposed to these test conditions.Specimen 3-c showed relatively good retention of strength in oxidizingconditions. TABLE 2 CG Nicalon ™ Fiber reinforced composites withMulti-layer Interface Coatings on Non-woven Fiber. Bulk Open FlexuralDensity Porosity Strength Example g/cm³ % , MPa Comments 2a 2.10 10.8430.6 3 pt. RT test as made condition 2b 2.12 10.6 318.1 3 pt. RT testafter 24 H₂O Boil 3a 2.16 2.5 478.2 3 pt. RT test as made condition 3b2.13 2.9 601.7 3 pt RT test after 24 H₂O Boil 3c 2.14 5.4 369.8 4 Pt1000° C. test (no aging)

Example 4

[0061] Specimens prepared in Example 4 were molded using 8 harness satinwoven cloth. The processing was done in a manner similar to Example 1except that the molding thermal cycle also included a higher temperaturecure step of 300° C. for 4 hours. In addition, some of the panels werealso subjected to a one-time thermal treatment at the fourteenth orfifteenth pyrolysis cycle to 1300° C.

[0062] Table 3 lists the filler type, the number of pyrolysis cyclesused to densify the composite panels and a designation for the interfacecoating. A description of the “Mods” is listed in Table 4. Table 3 alsolists four point flexure strengths in the as made connection, at 1100°C. and after heating specimens at 1100° C. for 50h in air.

[0063] The asterick (*) specimens listed in Table 3 were panels preparedwith a combined unitape and 8 harness satin woven cloth architecture.The plies of the composite were 0/90/0/8HS/8HS/8HS/8HS/0/9010. Thissymmetrical architecture used three tape plies on the outsides of fourplies of 8 harness satin woven cloth.

[0064] Samples F, G and J-M are for comparison. In general, themulti-layer interface coatings perform as well as or better than the BNor BN/SiC interface coated specimens in flexure tests. TABLE 3 Processand Properties of CG Nicalon ™ Fiber Reinforced Ceramic MatrixComposites Four Pt Flexural Strength, MPa # Cycles Process Room R.T.after 50 h Sample Interface Filler Densification Temp ° C. Temp. @ 1100° C. @ 1100 ° C. A Mod 1 Si₃N₄ 15 1200 594.8 531.3 618.2 B Mod 1 Si₃N₄17 1300 492.9 480.2 487.8 C Mod 2 Si₃N₄ 17 1300 443.0 546.5 468.5 D Mod2 Si₃N₄ 17  1200* 464.4 572.0 474.0 B Mod 2 Si₃N₄ 17  1300* 499.6 565.1464.4 F Mod 3 Si₃N₄ 15 1200 330.5 394.0 457.5 G Mod 3 Si₃N₄ 17 1300323.6 402.3 391.2 H Mod 4 Si₃N₄ 15 1200 373.3 356.0 347.8 I Mod 4 Si₃N₄17 1300 411.9 527.8 478.2 J BN BN 15 1200 389.8 426.4 573.4 K BN BN 171300 349.8 393.3 496.8 L BN Si₃N₄ 15 1200 348.4 436.8 364.3 L BN Si₃N₄17 1300 280.1 267.7 479.6 M BN None 17 1300 248.4 285.0 193.9

[0065] TABLE 4 Description of various binary interface coatings Mod 1 =1 binary coating of 0.3 μm BN, 0.2 μm Si₃N₄ Mod 2 = 2 binary coatings of0.05 μm BN, 0.05 μm Si₃N₄ Mod 3 = 2 binary coatings of 0.05 μm BN, 0.05μm SiC Mod 4 = 5 binary coatings of 0.02 μm BN, 0.02 μm Si₃N₄

Example 5

[0066] Panels prepared as in Example 4 were subjected to interlaminarshear strength tests before and after a rain and simulated enginethermal cycle exposure for fourteen and twenty-eight days. This combinedexposure to water and thermal stress evaluates the durability of CMCparts for some aerospace applications. The test consisted of tensilefatigue of specimens to 69 MPa (10 ksi) at room temperature at 10⁴fatigue cycles prior to the rain exposure. Specimens were then subjectedto simulated rain at the rate of 0.254 cm per day (approximately 2minutes). After the rain simulation, specimens were stored at 90° F. and90% relative humidity for approximately 22 hours and then subjected to asimulated Engine Thermal Cycle (see Table 5).

[0067] The results are shown in FIGS. 1 and 2. FIG. 1 shows the resultsfor fibers coated with only BN. FIG. 2 shows the results for fibers withthe various MOD coatings. TABLE 5 Engine Thermal Cycle Temperature Time(min) (° C.) 4 135 1 925 17  450 3 925 5 450 2 925 4 450 26  290 8 135end cycle room temp.

What is claimed is:
 1. A method of forming a fiber-reinforced ceramicmatrix composite comprising: (a) impregnating a ceramic fiber coatedwith at least one layer binary coating comprised of boron nitride andsilicon nitride wherein the silicon nitride is applied over the boronnitride with a preceramic composition comprising a curable preceramicpolymer; (b) forming the impregnated fibers into a desired shape; (c)curing the formed impregnated fibers; (d) heating the cured impregnatedfibers of (c) to a temperature of at least 1000° C. in an inertatmosphere for a time effective to convert the preceramic polymer to aceramic.
 2. The method of claim 1 wherein the cured impregnated fibersare heated in step (d) to a temperature of at least 1200° C.
 3. Themethod of claim 1 wherein the preceramic composition additionallycomprises fillers.
 4. The method of claim 1 wherein there is anadditional coating over the binary coating.
 5. The method of claim 1wherein the additional coating is silicon carbide.